The present invention relates to a container comprising a foamed olefin resin, and a process for producing same.
Although a container comprising a foamed olefin resin is extensively used for articles such as detergents, medicines, drinks and foods, it is desired to develop a container comprising a foamed olefin resin, which container is light in its weight per content volume.
It is an object of the present invention to provide a container comprising a foamed olefin resin, which container is light in its weight per content volume, and has enough rigidity to use practically.
It is another object of the present invention to provide a process for producing such a container.
The present invention provides a container comprising a foamed olefin resin, wherein weight W (g) and content volume V (ml) of the container satisfy the following expression (1):
xe2x80x83W(g)/V(ml)xe2x89xa60.055xe2x80x83xe2x80x83(1).
The present invention also provides a process for producing a container comprising a foamed olefin resin, which comprises the steps of:
(1) extruding a composition comprising an olefin resin and an expanding agent with an extruder to form a parison comprising a foamed olefin resin,
(2) setting the parison in a mold having a shape of the container, which mold is installed in a blow molding machine,
(3) blowing a gas having a pressure of from 0.01 to 3 kg/cm2 to both the outside surface of the parison and the inside surface thereof to cool the surfaces of the parison,
(4) blowing a gas having a pressure of from 2 to 10 kg/cm2 to the inside of the parison to expand and cool the parison, until the outer wall of the parison reaches the inner wall of the mold, and
(5) taking out the resulting container comprising a foamed olefin resin from the mold.
The weight (W) and content volume (V) of the container in accordance with the present invention satisfy the above-defined expression (1), preferably satisfy the following expression (2), and more preferably satisfy the following expression (3).
0.012 xe2x89xa6W(g)/V (ml)xe2x89xa60.050xe2x80x83xe2x80x83(2)
0.017 xe2x89xa6W(g)/V (ml)xe2x89xa60.045xe2x80x83xe2x80x83(2)
A thickness of the body of the container in accordance with the present invention is preferably from 0.2 to 3 mm, and more preferably from 0.4 to 2 mm from a viewpoint of accomplishing more effectively the objects of the present invention.
An expansion ratio of the container in accordance with the present invention is preferably from 1.1 to 5 times, and more preferably from 1.2 to 4.5 times.
While rigidity of a container, namely toughness thereof, generally depends greatly upon a thickness of the body of the container, a container in accordance with the present invention has a higher rigidity than a non-foamed container having the same weight and volume as the container in accordance with the present invention, because the body of the container in accordance with the present invention is greater in its thickness.
The xe2x80x9colefin resinxe2x80x9d used in the present invention means a thermoplastic resin containing not less than 50% by mol of a structure unit derived from an olefin (hereinafter referred to as xe2x80x9colefin unitxe2x80x9d) and less than 50% by mol of a structure unit derived from a comonomer other than the olefin, which comonomer is copolymerizable with the olefin. Examples of the olefin resin are olefin homopolymers, copolymers containing at least two olefin units only, and copolymers containing at least one olefin unit and a structure unit derived from a comonomer other than the olefin, which comonomer is copolymerizable with the olefin.
Preferred examples of the olefin are ethylene; an xcex1-olefin having 3 to 12 carbon atoms such as propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene and 1-decene; and a vinylcycloalkane such as vinylcyclohexane and vinylcyclopentane.
Examples of the comonomer other than the olefin are a conjugated diene such as butadiene and isoprene; a non-conjugated diene such as 1,4-pentadiene; acrylic acid; an acrylic acid ester such as methyl acrylate and ethyl acrylate; methacrylic acid; a methacrylic acid ester such as methyl methacrylate and ethyl methacrylate; and vinyl acetate.
Preferred olefin resins are those having a propylene unit. Of these, a propylene homopolymer and a block copolymer of propylene and ethylene are preferred.
A particularly preferred propylene homopolymer is that (hereinafter referred to as xe2x80x9cpropylene resin (A)xe2x80x9d, cf. JP-A 11-228629) obtained by a process comprising the steps of:
(1) homopolymerizing propylene to obtain a crystalline polymer (a) having an intrinsic viscosity of not less than 5.0 dl/g, and
(2) homopolymerizing propylene in the presence of the polymer (a) obtained in the above step (1) to further produce a crystalline polymer (b) having an intrinsic viscosity of less than 3.0 dl/g, thereby obtaining the propylene resin (A).
Here, it is important to satisfy the following conditions (i) to (iii) at the same time:
(i) an amount of the polymer (a) contained in the propylene resin (A) is from 0.05 to 25% by weight, provided that an amount of the propylene resin (A) is assigned to be 100% by weight,
(ii) an intrinsic viscosity of the propylene resin (A) is less than 3.0 dl/g, and
(iii) a molecular weight distribution of the propylene resin (A) is less than 10.
The above-mentioned xe2x80x9cblock copolymer of propylene and ethylenexe2x80x9d means a copolymer obtained by a process comprising the steps of:
(1) polymerizing propylene or a mixture of propylene and ethylene to obtain a propylene homopolymer or propylene/ethylene copolymer containing an ethylene unit of from 0 to 2% by weight, wherein an amount of the propylene homopolymer or propylene/ethylene copolymer is from 70 to 90% by weight, provided that an amount of the desired block copolymer of propylene and ethylene is assigned to be 100% by weight, and
(2) copolymerizing propylene and ethylene in the presence of the homopolymer or copolymer obtained in the above step (1) to further produce a propylene/ethylene copolymer containing an ethylene unit of from 10 to 50% by weight, thereby obtaining the block copolymer of propylene and ethylene.
A preferred combination of the olefin resin used in the present invention is (i) a combination of from 1 to 100% by weight of the above-mentioned propylene resin (A) with from 99 to 0% by weight of the above-mentioned block copolymer of propylene and ethylene containing an ethylene unit of from 1 to 10% by weight, and (ii) a combination of from 1 to 100% by weight of the above-mentioned propylene resin (A) with from 99 to 0% by weight of the propylene homopolymer, provided that the sum of two components in each combination is 100% by weight.
A container in accordance with the present invention can be produced by a process comprising the steps of:
(1) extruding a composition comprising an olefin resin and an expanding agent with an extruder to form a parison comprising a foamed olefin resin,
(2) setting the parison in a mold having a shape of the container, which mold is installed in a blow molding machine,
(3) blowing a gas having a pressure of from 0.01 to 3 kg/cm2 to both the outside surface of the parison and the inside surface thereof to cool the surfaces of the parison,
(4) blowing a gas having a pressure of from 2 to 10 kg/cm2 to the inside of the parison to expand and cool the parison, until the outer wall of the parison reaches the inner wall of the mold, and
(5) taking out the resulting container comprising a foamed olefin resin from the mold.
The formation of the parison in the above step (1) is carried out at a temperature higher than a higher temperature between a melting point of the olefin resin and a decomposition temperature of the foaming agent. When said temperature is too high, a melt viscosity of the olefin resin and a viscosity of the parison obtained decrease, respectively, and as a result, the resulting cells in the parison are crushed by pressure of the gas in the succeeding step (4), and therefore it is not recommendable. In the step (1), it is permitted to blow a pressure gas into the inside of the parison to prevent deformation of the parison.
The above-mentioned step (3) is a step of cooling the surfaces of the parison by blowing a pressure gas to both the outside surface of the parison and the inside surface thereof to prevent the resulting cells in the parison from being crushed by pressure of the gas in the succeeding step (4). It is permitted to cool the outside surface of the parison by the pressure gas in the above-mentioned step (1) as well as the step (3). It is preferable to cool the whole surface of the parison uniformly. It is not preferable to cool the parison unevenly, because a thickness of the body of the container obtained may become uneven. It is not desired to use a gas of too low pressure, because, if so, the parison cannot be cooled sufficiently. It is not also desired to use a gas of too high pressure, because, if so, (1) the parison is extraordinarily cooled to cause solidification and interruption of expansion, (2) the parison is deformed, (3) a part of the parison is expanded to the inner wall of the mold to cause the cool solidification, or (4) a container having a rough surface is obtained. Preferred examples of the gas used in this step are air, nitrogen and carbon dioxide.
When pressure of the gas in the above-mentioned step (4) is too low, expansion of the parison becomes insufficient, and therefore it is not desired. Whereas, when pressure of the gas is too high, the resulting cells in the parison are crushed by pressure of the gas, and therefore it is not desired. Preferred examples of the gas used in this step are air, nitrogen and carbon dioxide.
An expansion ratio of the container obtained is preferably from 1.1 to 5 times, and more preferably from 1.2 to 4.5 times.
The expanding agent used in the present invention may be ones known in the art. Examples of the expanding agent are inorganic expanding agents such as ammonium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, sodium nitrite and sodium borohydride; azo expanding agents such as azodicarbonamide and azobisbutyronitrile; nitro expanding agents such as dinitrosopentamethylenetetramine and N,Nxe2x80x2-dimethyl-N,Nxe2x80x2-dinitrosoterephthalamide; hydrazine expanding agents such as p-toluenesulfonyl hydrazide and p,pxe2x80x2-oxybisbenzenesulfonyl hydrazide; semicarbazide expanding agents such as p,pxe2x80x2-oxybisbenzenesulfonyl semicarbazide. Of these, sodium hydrogencarbonate and azodicarbonamide are preferred. It is permitted to use a combination of at least two expanding agents mentioned above. A combination of 1 to 40% by weight of azodicarbonamide and 60 to 99% by weight of sodium hydrogencarbonate is particularly preferred, provided that the sum of said two components is assigned to be 100% by weight.
The expanding agent is used in an amount of usually from 0.01 to 10 parts by weight, preferably from 0.05 to 8 parts by weight, and more preferably from 0.1 to 5 parts by weight, based on 100 parts by weight of the olefin resin used. When the amount of the expanding agent is less than 0.01 part by weight, it may frequently happen that the expansion ratio becomes insufficient to make production of a light weight container difficult. Whereas, when the amount thereof exceeds 10 parts by weight, it may happen that the effect of the foaming agent is saturated to cause an extraordinary cost.
Usually, it is recommendable to use an expanding agent having a decomposition temperature of about 140 to about 180xc2x0 C. With respect to an expanding agent having a decomposition temperature exceeding about 180xc2x0 C., it is recommendable to use such an expanding agent in combination with an expanding auxiliary to lower the decomposition temperature to about 140 to about 180xc2x0 C. Examples of the expanding auxiliary are zinc oxide, zinc nitrate, basic zinc carbonate, zinc stearate, zinc phthalate, lead carbonate, urea and glycerol.
The expanding auxiliary is used in an amount of preferably from 0.1 to 30% by weight, and more preferably from 1 to 20% by weight based on 100% by weight of the sum of the expanding agent and the expanding auxiliary.
In order to obtain a container having a light weight and high rigidity, it is recommendable to use the expanding agent in combination with an expanding nucleating agent, thereby generating a large quantity of cells having a small diameter. Examples of the expanding nucleating agent are inorganic fillers such as talc, silica, mica, calcium carbonate, calcium silicate, barium sulfate, amino silicate and diatomaceous earth; resin beads such as polymethyl methacrylate and polystyrene beads having a diameter of not more than 100 xcexcm; and metal salts such as calcium stearate, magnesium stearate and zinc stearate. It is permitted to use the expanding nucleating agent in combination of two or more thereof.
The expanding nucleating agent is used in an amount of preferably from 0.01 to 20% by weight, more preferably from 0.1 to 15% by weight, and much more preferably from 0.3 to 10% by weight, based on 100% by weight of the olefin resin. When the amount of the expanding nucleating agent is less than 0.01% by weight, it may happen that the effect of the expanding nucleating agent cannot be obtained sufficiently. Whereas, when the amount thereof exceeds 10% by weight, it may happen that the effect of the expanding nucleating agent is saturated to cause an extraordinary cost.
The olefin resin may be used in combination with additives such as neutralizing agents, antioxidants, heat stabilizers, weather resisting agents, crystal nucleating agents, lubricants, ultraviolet ray absorbers, anti-static agents and fluorescent whitening agents; and coloring agents such as dyes and pigments, so long as the objects of the present invention are accomplished.
The container in accordance with the present invention can be subjected to, for example, silk screen printing, offset printing, shrink labeling, stretch labeling and in-mold labeling, thereby decorating the surface thereof. As such a decoration method, for example, there is enumerated a method wherein a label is placed in advance in the mold of the step (2) mentioned above, whereby the label can be stuck fast to the surface of the container. An in-mold labeled blow-molded container obtained by said method has no height difference between the surface of the label and that of the container to exhibit a superior appearance, and therefore it is a particularly preferred container.
The container in accordance with the present invention can be provided with a cap to be used as a capped container. In addition, the container can be cut at the body thereof to obtain a cup and a tray. The cup and tray can be provided with a cap, whereby they can be used as a preservation container.
The container such as a vessel and a bottle in accordance with the present invention can be used, for example, as containers for daily liquid necessaries such as shampoo, hairdressing liquids, cosmetics, detergents, disinfectants and bleaching agents; containers for liquid foods such as soft drinks, water, edible oil and seasonings; containers for solid foods such as frozen foods, precooked foods and dairy products; containers for microwave ovens; containers for other medicines and agricultural chemicals; and containers for liquids for industries.